1. Field of the Invention
The present invention relates to a resistor the surface of which is coated with an overcoat glass to cover its resistor pattern, an electron gun for a cathode-ray tube using the same and a method of manufacturing the resistor.
2. Description of the Related Art
In recent years, demand has increased for a high resolution in a television, a display and the like.
To this end, as shown in FIG. 1, for example, an electron gun 31 having a common electric field extended lens i.e. an Extended Field Elliptical Aperture Lens referred to as "EFEAL" structure was developed and has become commercial. See SID '97 DIGEST p347-350 (1997).
This electron gun 31 is comprised of, though not shown, three cathodes K for generating electron beams corresponding to three colors or a red R, a green G and a blue B; respective electrodes, that is, a first electrode G.sub.1, a second electrode G.sub.2, a third electrode G.sub.3, a fourth electrode G.sub.4, a fifth electrode G.sub.5, an intermediate electrode GM which will be explained later, a sixth electrode G.sub.6 and a convergence cup 35 for accelerating and controlling the electron beams. The gun 31 is attached with a resistor 32 substantially parallel to a longitudinal direction of the electron gun 31. In FIG. 1, reference numeral 33 designates a stem and reference numeral 34 denotes a stem pin.
This electron gun 31 with the EFEAL structure needs a new electrode for applying an intermediate voltage (for example, 14 kV) between a conventional focus voltage (for example, 6 kV) and an anode voltage (for example, 27 kV).
To this end, the intermediate electrode GM is provided between the sixth electrode G6 on an anode side and the fifth electrode G.sub.5, or the focus electrode. The electron gun 31 having the EFEAL structure is such that the fifth electric electrode G.sub.5, the intermediate electrode GM and the sixth electrode G.sub.6 have, though not shown, electric field correcting electrode boards therein, each of which has beam penetration apertures corresponding to the three electron beams, and each of the electrodes G.sub.5, GM, and G.sub.6 is shaped like a cylinder which is cross-sectionally elliptical.
Then, by applying the above-mentioned intermediate voltage of, for example, 14 kV to the intermediate electrode GM, the penetration of the electric field to the beam penetration apertures of the electric field correcting electrode board (not shown) of the intermediate electrode GM controls the shape of the electron beams and the convergence thereof, thereby making it possible to optimize them.
Here, a voltage which can be applied through the stem pin 34 to the electron gun from the outside of the cathode-ray tube is limited to about 10 kV or so due to a withstand voltage characteristic between the pins.
Therefore, in order to apply the intermediate voltage of, for example, 14 kV and the like to the intermediate electrode GM, the resistor 32 becomes indispensable for connecting the low voltage from the stem pin 34 with the high voltage on the anode side and then driving the same.
FIGS. 2A and 2B show the resistor 32 of the electron gun 31 in FIG. 1. The cross-sectional view of resistor 32 is shown in FIG. 2A and the plan view thereof is shown in FIG. 2B. The resistor 32 is formed in such a manner that a conductive film is coated on one surface of a ceramic substrate 36 made of, for example, alumina and the like with a predetermined pattern, printed and fired to form a resistor pattern 37.
Then, an overcoat glass 38 is formed on the resistor pattern 37 and on the rear surface of the ceramic substrate 36 in order to protect the resistor pattern 37. Thus, the resistor 32 is formed.
The resistor 32 thus formed is fitted to the electron gun 31 with its surface of the ceramic substrate 36, on which the resistor pattern 37 is formed, being on the side of the electron gun 31 and its surface on the opposite side being outside, that is, on the neck-glass side of the cathode-ray tube.
An anode voltage, for example, a high voltage of 25-32 kV or so is applied to a high voltage electrode portion 39 at the left end of the resistor 32 and an earth electrode portion 41 at the right end thereof is grounded or is connected to an outer-fitted resistor outside the cathode-ray tube.
In the electron gun 31 of FIG. 1, the high voltage electrode portion 39 is connected to the convergence cup 35, the earth electrode portion 41 is grounded through the stem pin 34 and an intermediate electrode portion 40 of the resistor 32 is connected to the intermediate electrode GM.
The above-mentioned resistor 32 comprises, for example, a so-called inner dividing resistor (IBR: Inner Breeder Resistor), an IMR (Inner Middle voltage breeder Resistor), an IFR (Inner Focus breeder Resistor) and the like, and is used for applying a convergence voltage to obtain a convergence characteristic of the electron gun for the cathode-ray tube, applying a focus voltage of the electron gun for the cathode-ray and further, is used as a focus controller of a television receiver and the like other than for applying the intermediate voltage to the above-mentioned intermediate electrode GM.
However, in the case of such a resistor 32, there generates a growth of dendrite due to ion migration of natrium while it is in operation, resulting in a phenomenon that a portion between the resistor pattern 37 is electrically conducted.
For example, as shown in FIG. 2B, on the interface between the overcoat glass 38 and the ceramic substrate 36, there generates a growth of a dendrite 42 from an edge portion of the overcoat glass 38 toward the resistor pattern 37.
The growth of the dendrite 42 can be explained as follows.
As shown in FIG. 3, a natrium atom is ionized from Na.sub.2 O which is contained in the overcoat glass, the ceramic substrate and the resistor pattern as an impurity, and hence there is generated a natrium ion Na.sup.+. This natrium ion Na.sup.+ causes the ion migration along an electric potential gradient and moves to a cathode side (a low electric potential side) K.
Further, on the cathode side K, it absorbs oxygen from oxides in surrounding portion and precipitates as a layer of sodium oxide Na.sub.2 O with the result that the dendrite 42 made of sodium oxide Na.sub.2 O grows from the cathode side K to the anode side (a high electric potential side) A.
When the growth of the dendrite 42 progresses while the cathode-ray tube is in operation, there occurs the above-mentioned electrical conductivity among the neighboring portions of the resistor pattern 37. For example, in the case of FIG. 2B, originally, the intermediate electrode portion 40 applies 14 kV, but because a substantial resistor becomes short due to the short-circuit of the resistor pattern 37 on the low voltage side, the electric potential at the intermediate electrode GM rises up to a voltage of 15 kV and the like, thereby giving rise to a defective focus.
Particularly in recent years, there is a growing demand that a resistor be used under electrically and mechanically severe conditions like the above-mentioned FEEAL-type electron gun 31 and the like, thereby making the conductivity problem among the resistor pattern 37 serious.
Also, when miniaturization of the electron gun 31 is implemented, because the space among the resistor pattern 37 becomes narrow and the short-circuit tends to occur easily, it has been difficult to miniaturize the electron gun 31.